Procedures for surveying and navigation have changed markedly over the last 20 years, due to improvements in computer processing and the availability of location determination systems, such as GPS, GLONASS and LEO, although the goal of a survey remains the same: to provide precise location coordinate information for selected locations in a region.
Satellite-based location determination (LD) technology is used extensively for navigation, for mapping and for determination of survey locations, among other uses. The accuracy of an LD technology, such as GPS, is limited by errors due to activation of selective availability (SA), additional time delay due to propagation of signals in the ionosphere and the troposphere, satellite LD signal source timing errors, receiver timing errors, receipt of multipath signals and other similar sources.
Differentially corrected GPS (DGPS) technology had been developed to compensate for these error sources, especially the first three of these sources. DGPS technology uses a reference station receiver whose location is precisely known and which compares the theoretical pseudorange and/or carrier phase measurements it should receive, based on its known location, with the measurements actually made on the GPS signals received at the reference station. The differences between the theoretical measurements and the actual measurements are broadcast for use by other nearby GPS mobile receivers or rovers, in order to approximately correct for the measurements made at these rover stations.
In many applications, one limiting factor is the need for a mobile receiver to remain within a line-of-sight from the reference station, in order to receive the DGPS signal with low bit error rate. For example, in a terrain with mountains or with a heavy forest, high radiofrequency signals (above 100-300 MHz) cannot be received without risk of fading unless line-of-sight communication is used. Where the mobile receiver and reference station are separated by a distance of the order of 10 kilometers (km) or more, it is often difficult to ensure uninterrupted line-of-sight communication. This is often true in navigation applications and in surveying applications.
The prior art discloses use of a satellite-based location determination (LD) system for determining and displaying location coordinates of a selected location in a survey or for a navigation system. Prior art devices often involve use of a base survey receiver or station, which is preferably stationary during the survey and whose location is known very accurately, and of a mobile survey receiver or station whose location is estimated using LD signals received from several LD signal sources that are spaced apart from the mobile survey receiver. However, in some situations, more than two receivers (e.g., a mobile receiver and more than one rover receiver) are needed, because the mobile receiver cannot conveniently communicate with the base receiver using only line-of-sight signal transmission.
A total station survey instrument includes a visual sighting tool with instruments for distance measurement and angular orientation measurement. Recent improvements have incorporated a location determination system that can be used to more accurately determine the location, and possibly the angular orientation, of the survey instrument. Several limitations exist in use of a conventional total survey station. First, it is difficult to quickly establish the angular orientation and absolute location of a local survey or datum. Many surveys are not related to a uniform datum but exist only on a localized datum. In order to accurately orient a survey to a global reference, such as astronomical north, a star observation for azimuth is often used that requires long and complicated field procedures. Second, if a survey is to be connected to a national or state geodetic datum, the survey sometimes must be extended long distances, often tens of kilometers or more, depending upon the proximity of the survey to geodetic control marks. Third, the electronic total survey station relies upon line-of-sight contact between the survey instrument and the rodman or pole carrier, which can be a problem in undulating terrains. Often, a sequence of links must be used to complete a survey, and the possibility for introducing errors increases with the number of links used.
The systems disclosed by background patents in these activities provide some, but not all, of the benefits of a linked or chained Satellite Positioning System (SATPOS) integrated with a terrestrial total station instrument. What is needed is a system that: (1) provides precise location coordinates for the endpoints of a separation vector between a reference station and a rover or mobile station; (2) provides approximate location coordinates of a moving reference station relative to a fixed reference station; and (3) optionally includes distance- and angle-measuring sensors to aid in rapid determination of the separation vector between reference station and rover station(s).